Color and gray value digital images are both composed of picture elements (pixels), where each pixel is represented by multiple binary bits that define either a color or a gray level. In order to represent such an image on a bi-level printer, the individual color or gray level pixels are typically converted to binary level pixels through use of a digital halftoning process.
Digital halftoning is the process of transforming a continuous-tone image into a lower bit-depth, typically binary, image that has the illusion of the original continuous-tone image, using a careful arrangement of lower bit-depth picture elements. The process is also referred to as spatial dithering. In the case of color images, the color continuous-tone image is typically separated into color channels first. Separate halftones are then formed for each of the color channels.
Typically, for laser printers, ordered cluster dot halftones using lower lines per inch (lpi), such as 100-150 lpi, are best for photographs, areas of constant gray scale, or gradient patterns. Halftones using a lower lpi reduce print artifacts, such as banding, but may result in jagged edges for the sharp edges found in text and line art. Halftones using a higher lpi, such as 200-300 lpi, are best for text and line art, but are not as good for photographs, areas of constant gray scale, and gradient patterns. Print artifacts, such as banding, become more pronounced as the lpi is increased.